This invention relates to processes for the production of polymer sheet materials from oriented olefin polymer fibers and to the products of such processes.
GB 2253420B describes a process whereby an assembly of fibers of an oriented polymer may be hot compacted to form a sheet having good mechanical properties. The process involves an initial processing step in which the fibers are brought to and held at the compaction temperature whilst subject to a pressure sufficient to maintain the fibers in contact, the contact pressure, and thereafter compacted at a higher pressure for a few seconds, the compaction pressure. In the process a proportion of the fibre surfacesxe2x80x94most preferably from 5 to 10% by weightxe2x80x94melts and subsequently recrystallises on cooling. This recrystallised phase binds the fibers together. Preferred materials for use in this process are homo- and co-polymers of polyethylene.
The process of GB 2253420B can be used to produce complicated and precisely shaped monolithic articles having high stiffness and strength, and good energy-absorbing properties. However, a drawback of this process is the criticality of the compaction temperature, especially for polyethylene. This is shown by Comparative Example A in GB 2253420B.
In accordance with the present invention there is provided a process for the production of a monolithic article in which process an assembly of fibers of an oriented polyolefin polymer is subjected to a compaction process wherein the assembly of fibers is maintained in intimate contact at an elevated temperature sufficient to melt a proportion of the polymer, and is compressed, characterised in that prior to the compaction process the fibers have been subjected to a crosslinking process.
In some embodiments (referred to herein as xe2x80x9c2-step compactionsxe2x80x9d) the compaction process may comprise two distinct steps, namely a step of maintaining the assembly of fibers in intimate contact at an elevated temperature sufficient to melt a proportion of the fibre at a first, contact, pressure, and a subsequent compression step wherein the assembly is subjected to a second, compaction, pressure, higher than the contact pressurexe2x80x94as in GB 2253420B.
In some embodiments (referred to herein as xe2x80x9c1-step compactionsxe2x80x9d) the compaction process may comprise a single step of maintaining the assembly of fibers in intimate contact at an elevated temperature sufficient to melt a proportion of the fibre, and at a given pressure. In such embodiments there is no subsequent step of applying a higher pressure.
Preferably the monolithic article is an article which is shape stable under its own weight, such as a plaque.
The crosslinking process may be a chemical crosslinking process, involving the use of a chemical reagent which forms reactive radicals under predetermined initiation conditions. Suitably the reagent may be a cumene compound, or a peroxide, for example DMTBH or DCP, or a silane, for example a vinyl silane, preferably vinylmethoxy silane.
The crosslinking process may be an irradiation crosslinking process involving an ionising step comprising irradiating the fibers with an ionising radiation, and then an annealing step comprising annealing the irradiated polymer at an elevated temperature.
For general information on known crosslinking processes, reference may be made to Sultan and Palmlxc3x6f, xe2x80x9cAdvances in Crosslinking Technologyxe2x80x9d, Plast. Rubb. and Comp. Process and Appl., 21, 2, pp. 65-73 (1994), and to the references therein.
Irradiation crosslinking is believed to be particularly suitable, for the process of the present application.
The pre-compaction process of crosslinking has been found to increase the xe2x80x9ctemperature windowxe2x80x9d available for the subsequent compaction stage, and thus to make the compaction stage much easier to control. Further, compacted products produced by the process of the present invention have exhibited superior hot strength properties, compared with compacted products made from fibers which have not been subject to prior crosslinking.
The term xe2x80x9cfibersxe2x80x9d is used herein in a broad sense to denote strands of polyolefin polymer, however formed. The fibers subjected to prior crosslinking may be non-woven fibers laid in a web, or may be comprised within yarns, or constituted by bands or fibrillated tapes, for example formed by slitting films. If comprised within yarns or constituted by bands or fibrillated tapes, those yarns, bands or fibrillated tapes may be laid together or they may be formed into a fabric, for example by weaving or knitting.
Suitably the fibers used in the process of the invention are formed from molten polymer, for example as melt spun filaments.
Preferably the fibers used in the present invention have a weight average molecular weight in the range 10,000 to 400,000, preferably 50,000 to 200,000.
The polyolefin polymer can be selected from polyethylene, polypropylene or polybutylene, or copolymers comprising at least one of those olefin polymers. The polyolefin polymer used in the process of the present invention may suitably be a polypropylene homopolymer or a copolymer containing a major proportion of polypropylene. Advantageously it may be a polyethylene homopolymer or a copolymer containing a major proportion of polyethylene.
A polyethylene copolymer comprising a major proportion of polyethylene as defined herein is one comprising more than 50% by weight of polyethylene. Preferably, it comprises more than 70% by weight of polyethylene, most preferably, more than 85% by weight of polyethylene.
A polyethylene polymer as defined herein may be unsubstituted, or substituted, for example by halogen atoms, preferably fluorine or chlorine atoms. Unsubstituted polyethylene polymers are however preferred.
A polyethylene copolymer comprising a major proportion of polyethylene may have one or more different copolymers, following copolymerisation of ethylene with, for example, one or more of propylene, butylene, butadiene, vinyl chloride, styrene or tetrafluoroethylene. Such a polyethylene copolymer may be a random copolymer, or a block or graft copolymer. A preferred polyethylene copolymer is a ethylene-propylene copolymer, having a major proportion of polyethylene and a minor proportion of polypropylene.
A polypropylene copolymer comprising a major proportion of polypropylene as defined herein is one comprising more than 50% by weight of polypropylene. Preferably, it comprises more than 70% by weight of polypropylene, most preferably, more than 85% by weight of polypropylene.
A polypropylene polymer as defined herein may be unsubstituted, or substituted, for example by halogen atoms, preferably fluorine or chlorine atoms. Unsubstituted polypropylene polymers are however preferred.
A polypropylene copolymer comprising a major proportion of polypropylene may have one or more different copolymers, following copolymerisation of propylene with, for example, one or more of ethylene, butylene, butadiene, vinyl chloride, styrene or tetrafluoroethylene. Such a polypropylene copolymer may be a random copolymer, or a block or graft copolymer. A preferred polypropylene copolymer is a propylene-ethylene copolymer, having a major proportion of polypropylene and a minor proportion of polyethylene.
It is essential in the practice of the present invention that the process employs fibers which have been subjected to a crosslinking process. However, the co-use of a polymer component (not necessarily a polyolefin) which has not been subjected to a crosslinking process, and/or of an inorganic filler material, is not excluded.
A polymer which has not been subjected to a crosslinking process may, when present, be present in an amount up to 50 vol % of the total polymer content of the article. Preferably, however, substantially the entire polymer content of the article derives from polyolefin polymer which has been subject to a crosslinking process.
An inorganic filler material may be present. An inorganic filler, when present, may be present in an amount up to 60 vol % of the article, preferably 20 to 50 vol %. An inorganic filler material may, for example, be selected from silica, talc, mica, graphite, metal oxides, carbonates and hydroxides and apatite, for example hydroxyapatite, a biocompatible calcium phosphate ceramic.
The preferred crosslinking process, involving irradiation crosslinking, will now be further defined.
Preferably, the environment for the annealing step which follows irradiation is a gaseous environment.
Preferably the irradiation step is effected in an environment which is substantially free of oxygen gas. For example it could be performed in vacuo or in the presence of an inert liquid or gas. Preferably however the environment for the irradiation step comprises a monomeric compound selected from alkynes, and from alkenes having at least two double bonds.
Preferably, the annealing step which follows irradiation is carried out in an environment which is substantially free of oxygen gas but which comprises a monomeric compound selected from alkynes, and from alkenes having at least two double bonds.
There is no necessity for the environments to be the same, in the irradiation and annealing steps. Indeed there is no necessity for said monomeric compound used during the irradiation step to be the same as said monomeric compound used in the annealing step; the monomeric compound used in the irradiation step could be replaced in whole or in part by a different monomeric compound for the annealing step. However it is believed that the properties desired of said monomeric compound in the irradiation step will generally be the same as those required in the annealing step, so there will generally be no necessity to effect a whole or partial replacement. Most conveniently, therefore, the monomeric compound is the same throughout. In some cases however it may be advantageous to supply a further charge of said monomeric compound, as the process proceeds.
For either or both steps, a mixture of monomeric compounds could be employed.
The environment employed for the irradiation and/or the annealing steps is preferably constituted entirely by said monomeric compound, but may also comprise a mixture being said monomeric compound together with one or more other components, for example an inert gas or liquid. Suitably the said monomeric compound is gaseous at least under the treatment conditions employed and is employed in the irradiation and/or annealing steps at a pressure, or partial pressure in the case of a mixture, in the range of 0.2-4 atmospheres (2xc3x97104Pa-4xc3x97105Pa), preferably 0.5-2 atmospheres (5xc3x97104Pa-2xc3x97105Pa), most preferably 0.3-1 atmospheres (3xc3x97104Pa-1xc3x97105Pa).
Preferred monomeric compounds for use in the present invention, in either or both of the irradiation and annealing steps, are alkynes, and alkenes having at least two double bonds, which alkenes are not substituted by halogen atoms. They are desirably gaseous under the treatment conditions employed and should be able to diffuse into the polyolefin polymer under the treatment conditions employed. Preferred are unsubstituted alkynes or alkenes i.e. alkynes or alkenes made up substantially entirely by hydrogen and carbon atoms. Examples are unsubstituted C2-6 alkynes, preferably having only one triple bond, for example acetylene, methyl acetylene, dimethyl acetylene and ethyl acetylene (of which species acetylene is preferred) and unsubstituted C4-8 alkenes having at least two double bonds, preferably only two double bonds, for example 1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene, 1,4-hexadiene and 1,3,5-hexatriene (of which species 1,3-butadiene is preferred).
One preferred class of alkenes for use in the present invention has at least two conjugated double bonds, thus including 1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene and 1,3,5-hexatriene. Preferably the conjugation extends throughout the length of the compound, as is the case with 1,3-butadiene and 1,3,5-hexatriene.
Another preferred class of alkenes for use with the present invention has double bonds at least as the terminal bonds in the compounds, thus including 1,3-butadiene and 1,3,5-hexatriene.
A particularly preferred class of alkenes has at least two conjugated double bonds, preferably with the conjugation extending throughout the length of the compounds, and double bonds at least as the terminal bonds of the compounds. Compounds of this type thus include 1,3-butadiene and 1,3,5-hexatriene.
Preferably said alkyne or alkene having at least two double bonds is the sole crosslinking agent employed in the irradiation step and/or the annealing step.
Acetylene is an especially preferred monomeric compound for use in the present invention. Preferably acetylene is used as substantially the sole said monomeric compound both in the irradiation step and in the annealing step.
Suitably the irradiation step is effected at a temperature not exceeding 100xc2x0 C., preferably not exceeding 80xc2x0 C. A preferred range is 0-50xc2x0 C., most preferably 15-30xc2x0 C. Conveniently the step is effected at ambient temperature.
In carrying out the process of this invention, any ionizing radiation can be employed. In practice, however, the types of ionizing radiation which can be used with greatest practicality are electron beams, ultra-violet radiation and, especially, xcex3-rays.
The radiation dose is suitably in the range 0.5 to 100 MRads inclusive, preferably 1 to 50 MRads inclusive, most preferably 2 to 20 MRads inclusive. For many applications a radiation dose of 3 to 10 MRads inclusive appears very favourable.
Preferably the polyolefin polymer is annealed at a temperature of at least 60xc2x0 C., preferably at a temperature in the range 80 to 120xc2x0 C. inclusive.
Preferably the polyolefin polymer is annealed at an annealing temperature at least 20xc2x0 C. below its melting point, most preferably at an annealing temperature which is below its melting point by a temperature differential in the range 20 to 50xc2x0 C., inclusive, most preferably, 30 to 40xc2x0 C., inclusive.
The period for which annealing is carried out is not thought to be critical, provided that the time is sufficient for substantially all of the polymer which has been irradiated to reach the said annealing temperature and for substantially all of the radicals formed to have reacted. This can be assessed by trial and error following ESR or mechanical testing of irradiated and annealed samples; the presence of unreacted radicals is believed to lead to chain scission and diminution in mechanical properties.
Suitably the polyolefin polymer after the irradiation and annealing steps has a gel fraction at least 0.4, preferably at least 0.5. Preferably the polyolefin polymer after the irradiation and annealing steps has a gel fraction no greater than 0.85, preferably no greater than 0.75. A particularly preferred gel fraction is in the range 0.55 to 0.7, most preferably 0.6 to 0.65.
In accordance with the present invention there is provided a polyolefin polymer monolith prepared in accordance with the process of the invention, as defined above.
In relation to the compaction stage which follows the steps of irradiation and annealing, the description in GB 2253420B is in general applicable to the modified process of the present invention, for example in relation to treatment times, temperature, proportion of material which is to melt, the assembly of the fibers and molecular weights, and the description of GB 2253420B may be regarded as incorporated into the present specification by reference, insofar as it applies to the production of polyolefin articles. However, the preferred pressure conditions are different, and are set out below.
In 2-step compactions in accordance with the present invention the contact pressure is suitably in the range 0.01 to 2 MPa, preferably 0.1 to 1 MPa, most preferably 0.3 to 0.7 MPa; and the compaction pressure is suitably in the range 0.1 to 50 MPa, preferably 0.2 to 10 MPa, most preferably 0.3 to 7 MPa. In such 2-stage processes the compaction pressure should be higher than the contact pressure, suitably by a factor of at least 2, and preferably by a factor of at least 4.
In 1-step compactions in accordance with the present invention the (single) pressure applied is suitably in the range 0.1 to 10 MPa, preferably 0.2 to 5 MPa, most preferably 0.3 to 4 MPa.
It is especially preferred that the pressure, or maximum pressure, is such that the process may be carried out in an autoclave, or in a belt press or other apparatus in which the assembly is fed through a compaction zone in which it is subjected to said elevated temperature and said pressure.
A further difference between the present invention and that of GB 2253420B is that in the process of the invention the proportion of the polymer which melts is suitably at least 10% by weight, preferably 10 to 50% by weight.